So if I understand well the isolation condenser systems alone are sufficient to remove decay heat if we don't have a hole in the vessel or the pipes.

But if we have holes in the vessel and in the containment we absolutely need operator intervention to refuel the GDCS pools with active systems. So in this case we need operator action well before 72 hours to prevent a meltdown.

If we have a core meltdown, the Bimac core catcher take water from the GDCS pools to cool the molten core, so if your containment is intact you just need to refuel the IC pools and the PCCS pools. But if you have a containment failure you have to refuel the GDCS pools, and you can have hydrogen explosion (if there is air intake) and a lot of radiation release, am I correct ?

Yes, water makeup to the pools is needed in that extreme scenario, but not for the first 24 hours at least. Quite a bit of water is available for boiloff (way over a million liters!) and water boiloff takes a huge amount of energy. Plus both the isolation condensers and PCCS will offer some heat removal. Probably core damage will not occur for 36 hours at least but I'd have to recheck the numbers (was a long time ago that I ran the numbers on ESBWR).

Of course the normal shutdown cooling system and pool cleanup/cooling systems have water injection capability as well, both into the vessel and into the pools, and these are highly automated systems.

So you'd need vessel/piping damage plus containment damage plus a station blackout. And even then though it would be terminated by operators injecting the fire pump water.

ESBWR uses passive hydrogen recombiners so you never get a hydrogen explosion that damages your equipment.

Oh yes I forgot to mention there's also an equalization line connecting the suppression pool water to the vessel for passive injection. Taking that into account would push the core uncovery beyond 72 hours even with dual vessel and containment break and no operator action.

I knew that modern PWRs have passive hydrogen recombiners but i was wondering for BWRs because they have an inert containment. So after Fukushima I guess all new water reactors will have hydrogen recombiners.

I guess that with a pool type molten salt reactor (as you have discussed it in an other thread) with a lot of molten salts (for lot of heat capacity) you can claim a superior safety than a ESBWR, right ? Even with all pipes broken the molten salt of the pool will retain heat and non-volatile fission products.

It maybe does not need any operator action even with holes in your containment. If your pool salt is compatible with water you don't need worry about flooding. All you need is that there are not big cracks in your bed plate concrete to prevent big leaks of your pool.

Hydrogen recombiners can also be installed in inerted containments. They will start to work when hydrogen is around because there is some oxygen left even in inerted atmosphere (like 2% or so) and radiolytic decomposition will also generate oxygen. The recombiners are just beds filled with alumina pellets coated with palladium. The reaction is exothermic so it generates its own heated steam plume, this then produces natural circulation in the bed, sucking in more containment atmosphere with hydrogen and steam from the bottom.

Yes, a molten salt reactor can be designed to haver superior safety, due to the lack of chemical, pressurized steam, or other driving force reactions. It doesn't have to be designed to hold pressure as part of the passive containment cooling system, a safety and economic advantage over ESBWR/AP1000. It can also be designed to not require valves for emergency cooling, definately another advantage.

Leak of buffer salt won't be an issue, there's a lot of it and any leak into the ground will solidify and self plug the leak. In fact the salts are quite viscous and would tend to not get far into the ground.

Not all molten salt reactors are equal though. It is a very broad class of reactors and each type can be designed completely differently. So it is certainly possible to do stupid things that will incapacitate the cooling system. Molten salts do have inherent advantages of no stored energy and high boiling points that limit the release even when all cooling fails.

Thanks Cyril, the ESBWR is an impressive machine with an elegant design, maybe the best big LWR reactor on the market. GE can also claim to have a lot of expertise and feedback with BWRs. That helps to convince the public about the safety of this reactor. Sadly it's something that new types of reactor like MSRs can not claim. You can just learn general lessons from major accidents, but I guess it's not a big problem because today's designers are a lot more concerned about safety than 1960's designers.

I have an other question about pool type design MSRs : in case of the fuel salt boiling, do you think that the buffer salt can re-condense it or filtrate some of the actinides and fission products ?

Thanks Cyril, the ESBWR is an impressive machine with an elegant design, maybe the best big LWR reactor on the market. GE can also claim to have a lot of expertise and feedback with BWRs. That helps to convince the public about the safety of this reactor. Sadly it's something that new types of reactor like MSRs can not claim. You can just learn general lessons from major accidents, but I guess it's not a big problem because today's designers are a lot more concerned about safety than 1960's designers.

I have an other question about pool type design MSRs : in case of the fuel salt boiling, do you think that the buffer salt can re-condense it or filtrate some of the actinides and fission products ?

Thanks.

Every reactor today is safe. Even in fukushima nobody has died from radiation. The problem was that fukushima didn't have earthen levee tall enough to defend against tsunami. One report listed their levee as 5.7 meters or 18 feet high. The waves from the most powerful tsunami ever to hit land was 14 meters or 46 feet high. http://www.vgb.org/vgbmultimedia/News/F ... _rev16.pdf

At least one thousand people will die today from smoking tobacco.

Nuclear is safe. Smoking is not safe, ever, under any situations.

A 12 meter high earthen levee could have prevented ALL problems in fukushima far as I understand. Levee are really cheap, and last a long long time. (btw, I do not consider anything a levee unless it can be driven on top with a car or truck. If a levee cannot support a car or truck driving on top, it is not a levee. )

the boiling point of most molten salts being considered are >1400 °C.Never, could the salts boil in such a way. Air cooling alone would prevent it. Liquid fueled reactors do not have a single dense point of fuel, so there is no hot spots. Additionally the fuel will naturally slow down reactions as it starts to expand. This slow down and huge surface area of fuel to cool has a maximum temperature determined by simple physics.

Thanks NicholasJanssen, I know that nuclear power causes a lot much fewer deaths than other activities but peoples are very afraid by it, specially in the event where you have to evacuate a 1000 km2 land for decades. This is unacceptable for the public. Moreover, here in France, the public doesn't trust the industry and the state autorities anymore. This is due to bad communication and transparency from autorities about Tchernobyl accident and other accidents with our powerplants.

I think nuclear energy can regain public's acceptance by promoting a entirely new type of reactor like MSRs. But you have to convince them that there will be no big accidents even with extreme scenarios involving maybe fuel boiling.

I don't know, maybe we can imagine a reactivity insertion combine with a malfunction of the shutdown rods or a clog of the drain tank pipe. I know the salt fuel will heat up and kill reactivity but if it can not reach the drain tank in time or if you have failures with your DRACS or over passive decay heat removal, maybe it can boil. Then if you can mitigate this kind of scenario I think you have done your best in safety and the public will (maybe) be convinced.

After all one of the worst scenarios that solid fuel reactors must deal with is core meltdown. I think that in some states the regulatory autorities force the designers to consider core meltdown, even if there is no credible scenarios that involves it. What would be the equivalent for MSRs ? Maybe the boiling of the entire fuel salt, except if the regulatory autories are more friendly with MSRs.

I read that the Pebble Bed helium cooled reactors are extremely good for removing decay heat in extreme scenarios; they can remove heat by conduction in the walls because of their very low density power. I wonder if we can have the same level of safety with pool type MSRs where the buffer salt absorbs a lot amount of decay heat and then rejects it by conduction in the surrounding structures, but you can have problems with heat transfers from the fuel salt to the buffer salt, that why I am wondering what happens if you have a boiling of your fuel salt.

In some reactors it might be possible to get to slow boiling with decay heat. This would be a very slow heatup process and could be eliminated by design by having a cold surrounding (buffer salt or other passive cooling systems). The buffer salt would be designed to soak up enough heat so that fuel salt boiling does not occur, and the buffer salt will be designed to lose heat at a rate that is greater than the heat rate required to boil the buffer salt. So boiling can be eliminated by design.

If fuel salt leaks it would be dissolved into the buffer salt. Still no boiling, in fact the heat source is now diluted in a larger volume of buffer salt. Even volatile components like iodine will be diluted which reduces their volatility greatly. As long as the buffer salt doesn't boil there won't be a large release even with containment failure. And it's easy to make it so that the buffer salt doesn't boil, via heat loss.

Fuel salt boiling is not a credible accident scenario. The one to fuss over with MSRs is a major leak in the off-gas system. This is especially true if when xenon that decays to cesium we leave the cesium as elemental. Then the cesium would be relatively volatile. The simple solution is to be sure there is a flourine donor compound in the off-gas system so that as the xenon decays the cesium will immediately be locked up as CsF.Still the gases are very radioactive (for a short time) and we may run into problems with licensing if the offgases get collected into one place. If an MSR is going to spill radiation into the environment beyond the immediate grounds it will be through the off-gas system.

At least in Britain the argument is won over the 'safety' of nuclear power, the anti nuclear movement has given up attacking it on that basis.

They now attack entirely on a cost basis, that is where the battleground lies.

ESBWR is probably best positioned to win that fight in the short term.

I think that opponents of nuclear power are a bit smarter and more adaptable than that, they will hammer away on any angle where damage can be done; doom and destruction, safety of reactors, safety of spent fuel, proliferation risks associated with all aspects of the nuclear industry and nuclear technology, Fukushima Daichi, Chernobyl, Three Mile Island. Emphasis on all of this, driven by the politics of fear, fuelled by ignorance, is then reflected in over the top regulation and outrageously low regulatory limits, ALL of which drives up cost, delays and risk.

At least in Britain the argument is won over the 'safety' of nuclear power, the anti nuclear movement has given up attacking it on that basis.

They now attack entirely on a cost basis, that is where the battleground lies.

ESBWR is probably best positioned to win that fight in the short term.

I think that opponents of nuclear power are a bit smarter and more adaptable than that, they will hammer away on any angle where damage can be done; doom and destruction, safety of reactors, safety of spent fuel, proliferation risks associated with all aspects of the nuclear industry and nuclear technology, Fukushima Daichi, Chernobyl, Three Mile Island. Emphasis on all of this, driven by the politics of fear, fuelled by ignorance, is then reflected in over the top regulation and outrageously low regulatory limits, ALL of which drives up cost, delays and risk.

I agree Lindsay, anti-nukes will use every possible angle to discredit clean cheap safe nuclear energy...

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